Journal of Clinical and Experimental Neuropsychology
ISSN: 0168-8634 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/ncen19
Asymmetrical movement overflow in children depends on handedness and task characteristics Shelley E. Parlow To cite this article: Shelley E. Parlow (1990) Asymmetrical movement overflow in children depends on handedness and task characteristics, Journal of Clinical and Experimental Neuropsychology, 12:2, 270-280, DOI: 10.1080/01688639008400973 To link to this article: http://dx.doi.org/10.1080/01688639008400973
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Journal of Clinical and Experimental Neuropsychology 1990, Vol. 12, NO.2, pp. 270-280
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Asymmetrical Movement Overflow in Children Depends on Handedness and Task Characteristics* Shelley E. Parlow University of Calgary
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ABSTRACT Unintentional movement overflow between the hands was examined in 176 righthanded children in grades 2,4, and 6. A significant multivariate hand difference was observed for contralateral but not for ipsilateral overflow during rapid repetitive movements and discrete finger placement tasks. Individual ANOVAs revealed greater overflow from left to right hands during forearm pronation/ supination (a rapid repetitive task) but the reverse during finger-displacement tasks. Hand differences in contralateral overflow were also observed in 19 lefthanded children, with greater overflow from right to left hands observed for two tasks. These data support previous reports of asymmetrical overflow, but suggest that the presence and direction of the phenomenon depends on the nature of the intended movement and on handedness.
Unintentional movement “overflow” involving muscles superfluous to an intended action is a common observation in young children and has been identified by many names, including “synkinesis” and “associated movements.” For example, the intentional movement of one finger may be accompanied by unintentional movements involving other fingers of the same or the opposite hand. The frequency of these associated movements drops off dramatically between 5 and 8 years of age (Wolff, Gunnoe, & Cohen, 1983). They can be elicited in older children and adults, but the overt expression of associated movements in these subjects appears to be influenced by many factors, including gender (Connolly & Stratton, 1968), hand order of testing (Todor & Lazarus, 1983, as reported in Todor & Lazarus, 1985), functional characteristics of the intended movement (Wolff et al., 1983), amount of effort expended (Cernacek, 1961), and injury or developmental anomaly involving the central nervous system (Woods & Teuber, 1978). A more complete review is provided by Todor and Lazarus (1985). Another factor that has generated considerable interest is the side of the body
* Funding for this project was provided by an Alberta Heritage Foundation for Medical Research Fellowship. The author would like to thank B. Kaplan, W. Webster, and J. Murray for their helpful comments. Requests for reprints should be sent to: Dr. S. Parlow, Department of Psychology, Carleton University, Ottawa, Canada K1S 5B6. Accepted for publication: March 10, 1989.
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that makes the intended movement. Specifically, researchers have reported hand differences in the frequency of “mirror” movements involving homologous muscles of the opposite hand. These are more frequently observed when the left hand performs an intended movement than when the right hand does so. This phenomenon has been observed even when the movements require equivalent effort (Liederman & Foley, 1987; Todor & Lazarus, 1986). A number of theories have been suggested t o explain the finding. First, the hand difference might reflect putative differences in cerebral representation of sensorimotor functions in the left and right hemispheres. If these functions are more diffusely represented in the right hemisphere (cf. Semmes, 1968), greater overflow from the left to the right hand would be expected due to the greater potential for motor irradiation offered by right-hemisphere control of movement. Second, the hand difference might reflect involvement of the ipsilateral (left) hemisphere in controlling intentional movements of the left hand (cf. Kimura & Archibald, 1974). According to this view, use of the left hand activates both hemispheres, and might therefore lower movement thresholds in the right hand. As use of the right hand activates only the left hemisphere, overflow from the right to the left hand is not predicted by this theory. A third theory holds that differential use of the preferred hand leads to the development of more refined learned patterns of neuronal activity in the left hemisphere (Todor & Lazarus, 1985), which potentiates less motor irradiation. This view is similar to the first, but focuses on the left hemisphere’s role rather than that of the right. All three theories assume that the direction of greater overflow is invariant and that some property of one hemisphere (left or right) accounts for the phenomenon. A fourth alternative needs to be considered, however (Todor & Lazarus, 1985). Typically, the intended movements studied by researchers have relied on abilities associated with the left hemisphere, such as strength or sequencing ability. But what if hand differences in overflow reflect a more general property of the nervous system relating t o functional specialization? If so, and the intended movement were t o contain a substantial spatial or somesthetic component (abilities associated with the right hemisphere), one should see more mirror movements with right-hand activity. A detailed examination of the literature reveals several failures to replicate the reported left-hand effect (e.g., Connolly & Stratton, 1968; Hiscock, Decter, Kinsbourne, & Mackay, 1987; Wolff et al., 1983) in some or all movements studied. Furthermore, in two studies (Cernacek, 1961; Woods & Teuber, 1978), the opposite effect has been reported, that is, that mirror movements were more frequent with intentional movement of the right hand. Cernacek used an EMG methodology to examine associated movements in adults as they performed isolated finger flexions. His task is somewhat unusual in this literature, although isolated finger-displacement movements are often included in tests of associated movement. Kimura and Vanderwolf (1970) studied a similar flexion task and argued that, in contrast to most unimanual
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activities, this kind of task is better performed by the left hand (right hemisphere). Woods and Teuber found that hand differences in movement overflow (summed across a variety of tasks) reversed at around 14years of age. Their finding is difficult to explain. The age effect might be at least partially accounted for, however, if relative hemispheric contributions vary for acquired versus developing skills (cf., Goldberg & Costa, 1981). These findings present a problem for theories that hold hand differences in overflow to be invariant. The present study was designed to examine the role of hemispheric specialization of function in contralateral movement overflow,and specifically as it pertains to rapid, repetitive (left hemisphere) vs. isolated finger movement (right hemisphere) tasks. The tests of associated movement were included in a larger study of interhemispheric communication which focused on the separate issue of transfer of training between hands (Parlow & Kinsbourne, 1989). Boys and girls in three grades (2,4, and 6) were tested. It was hypothesized that, if some property of one hemisphere (left or right) accounts for the commonly reported pattern of greater left-hand overflow, this pattern should be evident in both types of movement. However, if the basis for the phenomenon lies with hemispheric specialization of function per se, it was hypothesized that hand differences would be task specific, with greater lefthand overflow evident for rapid repetitive tasks and greater right-hand overflow evident for finger placement tasks. Because the parameters of the phenomenon are not well defined, data were collected on both mirror (homologous muscle activity) and non-mirror (heterologous muscle activity) movements of the contralateral hand, and also on (heterologous) associated movements of the ipsilateral hand. Also, because little is known about associated movements in left-handers, data collected from left-handed children were analyzed separately. This population includes individuals with atypical patterns of brain organization (Segalowitz & Bryden, 1983). If hand differences in overflow are related to brain organization (involving one or both hemispheres), one might therefore expect this heterogeneity to mask hand differences. However, if related to preferential hand usage, one might expect left-handers to show greater right-hand overflow in both types of task (opposite to right-handers). METHOD Subjects A total of 195 children from three local schools volunteered to participate in the study. Handedness was determined by self-report and verified as the children pantomimed 15 unimanual tasks, including writing, drawing and using scissors. Right-handed children used the right hand for an average of 14.4 (SD= 1.2) of these tasks, left-handed children for an average of 11.3 (SD = 3.9). The sample included 176 right-handers:63 in grade 2 (36 boys, 27 girls), 55 in grade 4 (26 boys, 29 girls), and 58 in grade 6 (24 boys, 34 girls). The average age at testing was 7 years, 9 months (*6 months) in grade 2 , 9 years, 9 months (+ 5 months) in grade 4, and
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1 I years, 9 months (+ 7 months) in grade 6. The 19 left-handers included 6 in grade 2 (3 boys, 3 girls), 5 in grade 4 (1 boy, 4 girls), and 8 in grade 6 (7 boys, 1 girl). Their average ages were 7 years, 8 months (+ 8 months), 9 years, 8 months (5 3 months), and 12 years, 5 months (+ 8 months), respectively.
Procedure The children were tested in quiet rooms at their schools. They performed six movements (2 rapid repetitive and 4 finger-displacement) with each hand. The rapid repetitive movements (P and FTA, specified below) were performed while seated with both elbows resting on a table, forearms held upright (i.e., perpendicular to the table). The fingerdisplacement movements ( L M , L-R, SP1, SP2) were also accomplished with elbows on the table, but with palms down flat. Care was taken to ensure that the hands were relaxed and that the children did not press their fingers into the table, as this might introduce additional sensory cues associated with active resistance. Both hands were in vicw at all times. Hand order of testing was randomly assigned. Approximately half of the children in each grade were tested first with the right hand in each task, and half with the left hand first. A more detailed description of the required movements follows: 1. Forearm pronation/supination (P). With elbows on the table, the child was asked to make a fist with both hands and then to relax them. This produced a loosely fisted posture in the hands. The examiner then demonstrated the desired movement by rotating the fist in a horizontal plane, alternately supinating and pronating the forearm. She then pointed to one hand and asked the child to repeat the movement with this hand (and only this hand), as quickly as possible until asked to stop. Ipsilateral overflow on this task is usually seen as a generalized extension of the fingers on the designated hand from the fisted position. 2. Finger-thumb allernation (FTA). The examiner first dcmonstrated the movement by alternately touching the tip of the thumb to each fingertip on the same hand in the following order: thumb to index, middle, ring, little finger, ring, middle, index. The child was then asked to repeat this sequence with the designated hand until asked to stop, and to d o so “as fast as you can.” Ipsilateral overflow on this task is usually seen as stumbling or “jamming” of the fingers of the designated hand. 3. Finger-&firing.For this task, the fingers were slightly separated. Care was taken that the fingertips were not pressed into the table and that both hands were held in a natural position. After demonstrating a similar movement, the examiner touched the middle (L M) or ring ( L R ) finger and instructed the child to lift the designated finger (and only that finger) off the table. 4 .Finger-spreading. For this task, the fingers were held so that they barely touched. Again, care was taken to ensure that the hands were held in a natural position and that the fingertips were not pressed into the table. After demonstrating a similar movement, the examiner touched two adjacent fingers on one hand, either the index and middle fingers (SP1) or the middle and ring fingers (SP2), and instructed the child to spread these fingers (and only these fingers) apart to form a “V”. Scoring Ipsilateral and contralateral overflow movements were observed at firsthand by the examiner, and scored separately. As there is no evidence that a “big” overflow movement is more meaningful or reliable than a “small” one, a qualitative rather than a quantitative scale was used. Contralateral movements were scored on a 3-point scale,
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with 0 = no movement, 1 = nonspecific (heterologous) movement, and 2 = a specific mirror movement (such that homologous muscles moved substantially in synchrony with the intended movement). Ipsilateral (heterologous) movements were scored as absent (0) o r present (1). All results are presented relative to the intended (and not the mirroring) hand.
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RESULTS
The percentages of children in each grade demonstrating contralateral and ipsilateral associated movements (i.e., with scores > 0) are presented in Fig. 1. As expected, overflow was more common in younger children but was observed 70
E R
"
c
50
E N T
'Q LIP.
30 20 10
0
P
WA
L-U
L.R
6Pi
sp1
TASK
GPADE4
R C
50
E N T
40
30 20
10
TAM
Figure 1. The percent of right-handed children demonstrating contralateral (con) and ipsilateral (ips) associated movements as a function of grade, task and hand. L=left hand, R=right hand.
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at all ages. Ipsilateral overflow was observed more frequently than contralateral overflow in the finger-displacement tasks, but was relatively rare in the rapid repetitive tasks. Differences in sensitivity between tasks were also apparent, in that some movements were clearly more sensitive to overflow than others. To analyze the data, scores from all six movements were entered into a MANOVA, with sex, hand order, and grade included as between-subject factors, and hand (left, right) and side (ipsilateral, contralateral) as repeated measures. Univariate ANOVAS were examined when the omnibus F for an effect was significant. Given the low frequency of some scores, one might question whether the assumption of normality was justified. Nonnormality has negligable consequences on Type-I and Type-I1 error probabilities, however, if ns are large and nondirectional tests are employed (Glass, Peckham, & Sanders, 1972). Unless otherwise indicated, the effects reported below were significant at p < .001. Not surprisingly, the effects of grade, F(12,318) = 6.18, and side, F(6, 159) = 81.56, were confirmed by the analysis. The Grade x Side interaction was also significant,F(12,318) = 3.80,p< .05, reflecting the fact that for two tasks, Pand FTA, the improvement with age was apparent only in contralateral and not in ipsilateral overflow. As ipsilateral overflow was relatively rare for these tasks, this may simply be a floor effect. Several interactions involving hand order were revealed. A three-way interaction of Sex x Hand Order x Side, F(6,159) = 2 . 5 9 , ~ < .05, was significant only for SPl on univariate analysis. Boys who were tested first with the left hand were assigned higher contralateral overflow scores during this movement ( M = .56) than were boys who had been tested first with the right hand ( M = .26). Hand order did not play a role in ipsilateral overflow for boys (Lk= .34, RL = .35) and did not affect the performance of girls in either case. A two-way interaction of Sex x Hand Order was significant on univariate analysis only for SP2, F(6,159) = 2 . 3 9 , ~< .05. Again, boys tested first with the left hand received higher scores ( M = .63) than did boys tested first with the right hand ( M E .42). Girls did not show this order effect (Ms = .43 and .44, respectively). Finally, the interaction of Hand x Hand Order, 46,159) = 2 . 9 0 , ~< .05, was significant for task GM, with higher scores associated with the first hand tested, whether left (left hand = .21, right hand = .OS) or right (right hand = .17, left hand = .07). The two factors of primary interest were hand, F(6, 159)= 5.14, and the Hand x Side interaction, 4 6 , 159) = 5.74. Simple effects revealed hand differences in contralateral, F(6, 159) = 6.23, but not in ipsilateral ( F < 1) scores. Both main effect and interaction were significant at the univariate level for tasks P and SP2. For P, the average left-hand contralateral score ( M = .3 1) exceeded that of the right ( M = .lo), (simple effect: F(1,164) = 19.11). In contrast, the average righthand contralateral scorc for SP2 ( M = .44) exceeded that of the left ( M = .21), (simple effect: F(1, 164) = 13.08). Because of the importance of these findings, the scores were also subjected to the Wilcoxin Matched-Pairs Ranked-Signs (non-parametric) Test. Right-left difference scores wcrc ranked, ignoring signs.
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Then the sums of the ranks for positive and negative differences were calculated, and a test statistic 2 computed. This test confirmed that the hand differences obtained for P and SP2 were significant.
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Mirror vs. non-mirror contralateral overflow A breakdown of the frequency data for contralateral overflow into mirror (score = 2) and nonmirror (score = I) movements was then undertaken. The patterns of hand differences for P and SP2 were similar in both categories, and changed little across the three grades (see Table 1). The ratio of left to right hand overflow was 2.8: 1 for P, and 1:2.1 for SP2. Table 1. Frequency of mirror (score=2) and nonmirror (score=l) contralateral overflow in 176 right-handers. Grade 2 (n=63) Task: overflow type
Left
Grade 4
Grade 6 (n=58)
(n=55)
Right
Left
Right
Left
Right
Mirror(2) Nonmirror (1)
7 12
2 6
8 7
1 2
1 4
0 3
FTA: Mirror (2) Nonmirror (1)
12 24
15 29
11 15
7 19
1 5
3
L M : Mirror (2) Nonmirror (1)
5 0
4
2 0
1
3
0
0 0
1 0
LR: Mirror (2) Nonmirror (1)
6 4
I 2
2 0
2 1
0 0
1 2
SPl: Mirror (2) Nonmirror (1)
15 7
18 6
9 1
8 3
3 1
3 1
SP2: Mirror (2) Nonmirror (1)
8 6
9 13
1
6
I1 7
1 4
10
P:
8
4
Repetitive vs. finger-displacement tasks. Contralateral overflow scores were summed across the rapid repetitive tasks (P and FTA) and across the finger-displacement tasks (LM, L R , SP1, SP2). The two totals were submitted to a second MANOVA. In this analysis, hand was the only repeated measure. Two significant effects were revealed, one for grade, F(4, 326) = 13.00, and one for hand, F(2, 163) = 5.59, P < .005. Older children received lower scores on both measures, and hand differences were again evident (for finger-displacements, F(1, 164) = 7.62; for rapid repetitive movements, f'( 1,164) = 3.28, P = .072). As one would expect from the preceding analysis, higher scores were associated with the right hand for the former (left = .76, right = 1.06), and with the left hand for the latter (left = 34, right = .70). Visual examination of group means for individual tasks (Fig. 1) suggests that the second dependent measure reasonably represented performance across the
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four finger-displacement tasks. However, FTA did not conform to the expected pattern for a rapid repetitive task. In grade 2 (left = .76, right = .94) and in grade 6 (left = .12, right = .24), but not in grade 4 (left = .81, right = .60),the pattern of performance on this task was more suggestive of a finger-displacement task.
Left-handers The 19 left-handers were matched for grade, gender, and hand order with 19 right-handers randomly selected from the appropriate pool. A third MANOVA was conducted on this sample to investigate the effect of handedness. Three other variables (grade, hand, and side) were included in the analysis. Each task was entered as a separate dependent measure. Main effects for grade, F(12, 54) = 6.67, and side, F(6, 27) = 30.28, and an interaction between the two, F( 12, 27) = 26.00, were observed, confirming the results of the earlier analysis. Again, contralateral scores declined with age but ipsilateral scores did not. Ofgreater interest was the interaction of Handedness x Hand,F(6,27) = 2.86, p < .05. This was observed in the absence of a main effect for handedness ( F = 1.01) or hand ( F < 1). Univariate analyses revealed higher scores for P and L M with movement of the nondominant hand in both groups. Right-handers produced more overflow with left-hand movement (P: left = 1.3, right = .05; L M: left = .21, right = .05), and left-handers produced more overflow with righthand movement (for P: left = .05, right = .13; L M : left = ,18, right = .21). Left t o right hand ratios for P and SP2 were determined for the left-handed children. For task P, the frequency of left to right hand contralateral overflow was 1: 1.5, and for SP2,2.3: 1 (based on 2 instances of contralateral overflow with left hand movement, and 3 with right hand movement for P, 7 and 3 for SP2). Thus, although the hand difference for SP2 was not statistically reliable in this small sample, the data suggest a reversal for this task relative to P, as was observed for the right-handed children.
DISCUSSION Prior attempts to explain greater left-hand movement overflow by association with structural or functional hemispheric differences have typically ignored the several reports of greater right-hand movement overflow in this literature. This may have led to the premature assumption by many investigators that some property of one hemisphere accounts solely for the phenomenon. In the present study, a significant multivariate hand difference was observed for contralateral but not for ipsilateral overflow during rapid repetitive (left hemisphere) movements and discrete finger placement (right hemisphere) movements. Individual ANOVAs revealed greater overflow from left to right hands during forearm pronation/supination (a rapid repetitive task) and the reverse during finger-displacement movements. The latter was most evident for SP2, a finger-
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spreading task. The hand differences were seen in both mirror and nonmirror associated movements and were robust across the age range 7 to 11 years and across gender. Hand differences in contralateral overflow were also observed in a small sample of left-handed children. These children demonstrated greater right-hand overflow for two tasks (P and L M ) and greater left-hand overflow for SP2, although the latter was not statistically reliable. These findings are consistent with the view that both cerebral hemispheres contribute in a complementary fashion to movement, and (in an as yet unknown way) to contralateral overflow as well. Greater contralateral overflow appears to be associated with movement of the hand ipsilateral to the specialized hemisphere (e.g., the right hemisphere for SP2 and the left hemisphere for P in right-handers). This theory accommodates Cernacek's (1974) observation of greater right hand overflow for isolated finger flexions, a task which is similar to SP2 in that it requires careful finger positioning in a single, slow movement. Cernacek's finding derived from a careful study of EMG activity in adults, and cannot be accounted for by alternative theories currently available. Some problems for this interpretation should be considered. In the present study, reliable hand differences were not observed for all movements. Given the known sensitivity of measures of overflow to age and task effects (cf. Wolff et al., 1983),this may reflect a less than optimal sample size and/or the need for a more sensitive measure of overflow. It is more troublesome that a second rapid repetitive task -- finger-thumb alternation -- did not show the pattern of hand differences (even in lesser form) seen in the pronation/supination task. In fact, the pattern of scores for this task was more similar to SP2 than to P. Spatial elements in the task may have proven more problematic for the children than the requirements of rapid execution. Many of the children found it difficult to bring the fingers to one point in space and/or to determine the finger to move next. Given the close proximity and visual similarity of the fingers, these elements may require the use of internally generated relational cues (provided by the right hemisphere). Alternatively, some other movement classification scheme (based, for example, on proximal versus distal musculature, or perhaps on task familiarity or automaticity) may better predict the direction of greater overflow. For example, the direction of overflow for the finger alternation task might reverse with extended practice, as the sequence becomes automatized (and thereby increasing the relative contribution of the left hemisphere, cf. Goldberg & Costa, 1981). If the distinction is proximal/ distal, this manipulation should not affect the outcome. Inclusion of additional rapid repetitive tasks in future studies is needed. When the left-handed children were compared with a matched sample of right-handers, both samples showed greater contralateral overflow with movement of the nondominant hand for P and for L M , a finger-displacement task. Scores for L = M were strongly affected in the larger sample by hand order, such that greater overflow was typically associated with the first hand used,
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whether left o r right (see also Todor & Lazarus, 1985). This suggests that the hand difference for L-M was spurious, likely reflecting the fact that hand order was unevenly distributed in the small sample (due to random assignment). The absence of a hand difference for SP2 in the left-handed sample may be more important. Although a frequency comparison of left- and right-hand scores suggested that the direction of greater overflow reversed for tasks P and SP2 in left-handers as in right-handers, the hand difference for SP2 was not statistically reliable in the small sample. If one chooses to disregard the latter, the data support the preferential hand usage theory (i.e., that more overflow is associated with movement of the nondominant hand in this population). It is therefore possible that the pattern of overflow differs for left- and right-handers. Sample size will be a critical factor in any future investigation of this issue. The mechanism of asymmetrical overflow is still unclear. Why should there be greater contralateral overflow with movement of the hand ipsilateral to a specialized hemisphere? Perhaps the nonspecialized hemisphere (left or right) recruits or coactivates (Parlow & Kinsbourne, 1989)the specialized hemisphere, which results in a lowering of thresholds for movement of the other hand. The fact that cortical regions controlling hand movement in motor cortex are not directly (i.e. callosally) interconnected (Jenny, 1979), explains why heterologous and homologous muscles (cf. nonmirror and mirror movements) show the same directional effects. If use of the hand contralateral to thc specialized hemisphere does not lead to recruitment of the nonspecialized hemisphere, thresholds for movement of the other hand would be unaffected, and overflow between the hands less likely. This theory has some similarities to the second theory discussed in the introduction (cf. Kimura & Archibald, 1974), in that use of one hand may activate two hemispheres rather than one. However, it extends the principle to accommodate the specialized natures of both hemispheres. The co-activation theory can also account for the absence of observed hand differences in ipsilateral overflow. This finding presents a problem for theories that depend on the assumption that a more diffuse (or less refined) neural representation potentiates motor irradiation. To accommodate it, these theories must allow that distal (contralateral) brain regions are affected differentially and proximal (ipsilateral) brain regions are not (Liederman & Foley, 1987). The coactivation theory handles the problem easily, however, as recruitment (or not) of one hemisphere by the other should have little effect on cortical control of the active hand. The present findings suggest that hand differences in overflow vary with the nature of the intended movement and with handedness. A better understanding of these factors may well clarify a complex and difficult literature, and may also be expected to increase reliability for judgements of other neurological “soft signs” in the future (an issue not addressed here).
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REFERENCES Cernacek, J. (1961). Contralateral motor irradiation - cerebral dominance. Archives of Neurology, 4, 61-68. Connolly, K., & Stratton, P. (1968). Developmental changes in associated movements. Developmental Medicine and Child Neurology, 10, 49-56. Glass, G.V., Peckham, P.D., & Sanders, J.R. (1972). Consequences of failure to meet assumptions underlying the fixed effects analysis of variance and covariance. Review of Educational Research, 42, 237-288. Goldberg, E., &Costa, L.D. (1981) Hemispheric differences in the acquisition and use of descriptive systems. Brain andlanguage, 14, 144-173. Hiscock, M., Decter, M.H., Kinsbourne, M., & Mackay, M. (1987, February). Development of selective finger-movement control in normal children. Presented to the meeting of the International Neuropsychological Society, Washington, D.C. Jenny, A.B. (1979). Commissural projections of the cortical hand area in monkeys. Journal of Comparative Neurology, 188, 137-146. Kimura, D., & Archibald, Y. (1974). Motor functions of the left hemisphere. Brain, 92, 337-350. Kimura, D., & Vanderwolf, C.H. (1970). The relation between hand preference and the performance of individual finger movements by left and right hands. Brain, 93, 769774. Liederman, J., & Foley, L.M. (1987). A modified finger lift test reveals an asymmetry of motor overflow in adults. Journal of Clinical and Experimental Neuropsychology, 9, 498-510. Parlow, S.E., & Kinsbourne, M. (1989). Asymmetrical transfer of training between hands: Implications for inter-hemispheric communication in normal brain. Brain and Cognition 11, 98-1 13. Segalowitz, S.J., & Bryden, M.P. (1983) Individual differences in hemispheric representation of language. In S.J. Segalowitz (Ed.), Language functions and brain organization. (pp. 341-371). New York: Academic Press. Semmes, J. (1968). Hemispheric specialization: A possible clue to mechanism. Neuropsychologia, 6, 11-26. Todor, J., & Lazarus, J. (1985). Inhibitory influences on the emergence of motor competence in childhood. In L.D. Zaichowsky & C.Z. Fuchs (Eds.), Psychology of motor behavior: Development, control, learning andperformance (pp. 239-255). Ithaca, NY: Movement. Todor, J.I., & Lazarus, J.C. (1986). Exertion level and the intensity of associated movements. DevelopmentalMedicine and Child Neurology, 28, 205-212. Wolff, H., Gunnoe, E., & Cohen, C. (1983). Associated movements as a measure of developmental age. Developmental Medicine and Child Neurology, 25,417-429. Woods, B.T., & Teuber, H.L. (1978). Mirror movements after childhood hemiparesis. Neurology, 28, 1152-1158.
ISSN: 0168-8634 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/ncen19
Asymmetrical movement overflow in children depends on handedness and task characteristics Shelley E. Parlow To cite this article: Shelley E. Parlow (1990) Asymmetrical movement overflow in children depends on handedness and task characteristics, Journal of Clinical and Experimental Neuropsychology, 12:2, 270-280, DOI: 10.1080/01688639008400973 To link to this article: http://dx.doi.org/10.1080/01688639008400973
Published online: 04 Jan 2008.
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Journal of Clinical and Experimental Neuropsychology 1990, Vol. 12, NO.2, pp. 270-280
0168-8634/90/1202-0270$3.00 @
Swets 8~Zeitlinger
Asymmetrical Movement Overflow in Children Depends on Handedness and Task Characteristics* Shelley E. Parlow University of Calgary
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ABSTRACT Unintentional movement overflow between the hands was examined in 176 righthanded children in grades 2,4, and 6. A significant multivariate hand difference was observed for contralateral but not for ipsilateral overflow during rapid repetitive movements and discrete finger placement tasks. Individual ANOVAs revealed greater overflow from left to right hands during forearm pronation/ supination (a rapid repetitive task) but the reverse during finger-displacement tasks. Hand differences in contralateral overflow were also observed in 19 lefthanded children, with greater overflow from right to left hands observed for two tasks. These data support previous reports of asymmetrical overflow, but suggest that the presence and direction of the phenomenon depends on the nature of the intended movement and on handedness.
Unintentional movement “overflow” involving muscles superfluous to an intended action is a common observation in young children and has been identified by many names, including “synkinesis” and “associated movements.” For example, the intentional movement of one finger may be accompanied by unintentional movements involving other fingers of the same or the opposite hand. The frequency of these associated movements drops off dramatically between 5 and 8 years of age (Wolff, Gunnoe, & Cohen, 1983). They can be elicited in older children and adults, but the overt expression of associated movements in these subjects appears to be influenced by many factors, including gender (Connolly & Stratton, 1968), hand order of testing (Todor & Lazarus, 1983, as reported in Todor & Lazarus, 1985), functional characteristics of the intended movement (Wolff et al., 1983), amount of effort expended (Cernacek, 1961), and injury or developmental anomaly involving the central nervous system (Woods & Teuber, 1978). A more complete review is provided by Todor and Lazarus (1985). Another factor that has generated considerable interest is the side of the body
* Funding for this project was provided by an Alberta Heritage Foundation for Medical Research Fellowship. The author would like to thank B. Kaplan, W. Webster, and J. Murray for their helpful comments. Requests for reprints should be sent to: Dr. S. Parlow, Department of Psychology, Carleton University, Ottawa, Canada K1S 5B6. Accepted for publication: March 10, 1989.
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that makes the intended movement. Specifically, researchers have reported hand differences in the frequency of “mirror” movements involving homologous muscles of the opposite hand. These are more frequently observed when the left hand performs an intended movement than when the right hand does so. This phenomenon has been observed even when the movements require equivalent effort (Liederman & Foley, 1987; Todor & Lazarus, 1986). A number of theories have been suggested t o explain the finding. First, the hand difference might reflect putative differences in cerebral representation of sensorimotor functions in the left and right hemispheres. If these functions are more diffusely represented in the right hemisphere (cf. Semmes, 1968), greater overflow from the left to the right hand would be expected due to the greater potential for motor irradiation offered by right-hemisphere control of movement. Second, the hand difference might reflect involvement of the ipsilateral (left) hemisphere in controlling intentional movements of the left hand (cf. Kimura & Archibald, 1974). According to this view, use of the left hand activates both hemispheres, and might therefore lower movement thresholds in the right hand. As use of the right hand activates only the left hemisphere, overflow from the right to the left hand is not predicted by this theory. A third theory holds that differential use of the preferred hand leads to the development of more refined learned patterns of neuronal activity in the left hemisphere (Todor & Lazarus, 1985), which potentiates less motor irradiation. This view is similar to the first, but focuses on the left hemisphere’s role rather than that of the right. All three theories assume that the direction of greater overflow is invariant and that some property of one hemisphere (left or right) accounts for the phenomenon. A fourth alternative needs to be considered, however (Todor & Lazarus, 1985). Typically, the intended movements studied by researchers have relied on abilities associated with the left hemisphere, such as strength or sequencing ability. But what if hand differences in overflow reflect a more general property of the nervous system relating t o functional specialization? If so, and the intended movement were t o contain a substantial spatial or somesthetic component (abilities associated with the right hemisphere), one should see more mirror movements with right-hand activity. A detailed examination of the literature reveals several failures to replicate the reported left-hand effect (e.g., Connolly & Stratton, 1968; Hiscock, Decter, Kinsbourne, & Mackay, 1987; Wolff et al., 1983) in some or all movements studied. Furthermore, in two studies (Cernacek, 1961; Woods & Teuber, 1978), the opposite effect has been reported, that is, that mirror movements were more frequent with intentional movement of the right hand. Cernacek used an EMG methodology to examine associated movements in adults as they performed isolated finger flexions. His task is somewhat unusual in this literature, although isolated finger-displacement movements are often included in tests of associated movement. Kimura and Vanderwolf (1970) studied a similar flexion task and argued that, in contrast to most unimanual
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activities, this kind of task is better performed by the left hand (right hemisphere). Woods and Teuber found that hand differences in movement overflow (summed across a variety of tasks) reversed at around 14years of age. Their finding is difficult to explain. The age effect might be at least partially accounted for, however, if relative hemispheric contributions vary for acquired versus developing skills (cf., Goldberg & Costa, 1981). These findings present a problem for theories that hold hand differences in overflow to be invariant. The present study was designed to examine the role of hemispheric specialization of function in contralateral movement overflow,and specifically as it pertains to rapid, repetitive (left hemisphere) vs. isolated finger movement (right hemisphere) tasks. The tests of associated movement were included in a larger study of interhemispheric communication which focused on the separate issue of transfer of training between hands (Parlow & Kinsbourne, 1989). Boys and girls in three grades (2,4, and 6) were tested. It was hypothesized that, if some property of one hemisphere (left or right) accounts for the commonly reported pattern of greater left-hand overflow, this pattern should be evident in both types of movement. However, if the basis for the phenomenon lies with hemispheric specialization of function per se, it was hypothesized that hand differences would be task specific, with greater lefthand overflow evident for rapid repetitive tasks and greater right-hand overflow evident for finger placement tasks. Because the parameters of the phenomenon are not well defined, data were collected on both mirror (homologous muscle activity) and non-mirror (heterologous muscle activity) movements of the contralateral hand, and also on (heterologous) associated movements of the ipsilateral hand. Also, because little is known about associated movements in left-handers, data collected from left-handed children were analyzed separately. This population includes individuals with atypical patterns of brain organization (Segalowitz & Bryden, 1983). If hand differences in overflow are related to brain organization (involving one or both hemispheres), one might therefore expect this heterogeneity to mask hand differences. However, if related to preferential hand usage, one might expect left-handers to show greater right-hand overflow in both types of task (opposite to right-handers). METHOD Subjects A total of 195 children from three local schools volunteered to participate in the study. Handedness was determined by self-report and verified as the children pantomimed 15 unimanual tasks, including writing, drawing and using scissors. Right-handed children used the right hand for an average of 14.4 (SD= 1.2) of these tasks, left-handed children for an average of 11.3 (SD = 3.9). The sample included 176 right-handers:63 in grade 2 (36 boys, 27 girls), 55 in grade 4 (26 boys, 29 girls), and 58 in grade 6 (24 boys, 34 girls). The average age at testing was 7 years, 9 months (*6 months) in grade 2 , 9 years, 9 months (+ 5 months) in grade 4, and
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1 I years, 9 months (+ 7 months) in grade 6. The 19 left-handers included 6 in grade 2 (3 boys, 3 girls), 5 in grade 4 (1 boy, 4 girls), and 8 in grade 6 (7 boys, 1 girl). Their average ages were 7 years, 8 months (+ 8 months), 9 years, 8 months (5 3 months), and 12 years, 5 months (+ 8 months), respectively.
Procedure The children were tested in quiet rooms at their schools. They performed six movements (2 rapid repetitive and 4 finger-displacement) with each hand. The rapid repetitive movements (P and FTA, specified below) were performed while seated with both elbows resting on a table, forearms held upright (i.e., perpendicular to the table). The fingerdisplacement movements ( L M , L-R, SP1, SP2) were also accomplished with elbows on the table, but with palms down flat. Care was taken to ensure that the hands were relaxed and that the children did not press their fingers into the table, as this might introduce additional sensory cues associated with active resistance. Both hands were in vicw at all times. Hand order of testing was randomly assigned. Approximately half of the children in each grade were tested first with the right hand in each task, and half with the left hand first. A more detailed description of the required movements follows: 1. Forearm pronation/supination (P). With elbows on the table, the child was asked to make a fist with both hands and then to relax them. This produced a loosely fisted posture in the hands. The examiner then demonstrated the desired movement by rotating the fist in a horizontal plane, alternately supinating and pronating the forearm. She then pointed to one hand and asked the child to repeat the movement with this hand (and only this hand), as quickly as possible until asked to stop. Ipsilateral overflow on this task is usually seen as a generalized extension of the fingers on the designated hand from the fisted position. 2. Finger-thumb allernation (FTA). The examiner first dcmonstrated the movement by alternately touching the tip of the thumb to each fingertip on the same hand in the following order: thumb to index, middle, ring, little finger, ring, middle, index. The child was then asked to repeat this sequence with the designated hand until asked to stop, and to d o so “as fast as you can.” Ipsilateral overflow on this task is usually seen as stumbling or “jamming” of the fingers of the designated hand. 3. Finger-&firing.For this task, the fingers were slightly separated. Care was taken that the fingertips were not pressed into the table and that both hands were held in a natural position. After demonstrating a similar movement, the examiner touched the middle (L M) or ring ( L R ) finger and instructed the child to lift the designated finger (and only that finger) off the table. 4 .Finger-spreading. For this task, the fingers were held so that they barely touched. Again, care was taken to ensure that the hands were held in a natural position and that the fingertips were not pressed into the table. After demonstrating a similar movement, the examiner touched two adjacent fingers on one hand, either the index and middle fingers (SP1) or the middle and ring fingers (SP2), and instructed the child to spread these fingers (and only these fingers) apart to form a “V”. Scoring Ipsilateral and contralateral overflow movements were observed at firsthand by the examiner, and scored separately. As there is no evidence that a “big” overflow movement is more meaningful or reliable than a “small” one, a qualitative rather than a quantitative scale was used. Contralateral movements were scored on a 3-point scale,
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with 0 = no movement, 1 = nonspecific (heterologous) movement, and 2 = a specific mirror movement (such that homologous muscles moved substantially in synchrony with the intended movement). Ipsilateral (heterologous) movements were scored as absent (0) o r present (1). All results are presented relative to the intended (and not the mirroring) hand.
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RESULTS
The percentages of children in each grade demonstrating contralateral and ipsilateral associated movements (i.e., with scores > 0) are presented in Fig. 1. As expected, overflow was more common in younger children but was observed 70
E R
"
c
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E N T
'Q LIP.
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0
P
WA
L-U
L.R
6Pi
sp1
TASK
GPADE4
R C
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E N T
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TAM
Figure 1. The percent of right-handed children demonstrating contralateral (con) and ipsilateral (ips) associated movements as a function of grade, task and hand. L=left hand, R=right hand.
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at all ages. Ipsilateral overflow was observed more frequently than contralateral overflow in the finger-displacement tasks, but was relatively rare in the rapid repetitive tasks. Differences in sensitivity between tasks were also apparent, in that some movements were clearly more sensitive to overflow than others. To analyze the data, scores from all six movements were entered into a MANOVA, with sex, hand order, and grade included as between-subject factors, and hand (left, right) and side (ipsilateral, contralateral) as repeated measures. Univariate ANOVAS were examined when the omnibus F for an effect was significant. Given the low frequency of some scores, one might question whether the assumption of normality was justified. Nonnormality has negligable consequences on Type-I and Type-I1 error probabilities, however, if ns are large and nondirectional tests are employed (Glass, Peckham, & Sanders, 1972). Unless otherwise indicated, the effects reported below were significant at p < .001. Not surprisingly, the effects of grade, F(12,318) = 6.18, and side, F(6, 159) = 81.56, were confirmed by the analysis. The Grade x Side interaction was also significant,F(12,318) = 3.80,p< .05, reflecting the fact that for two tasks, Pand FTA, the improvement with age was apparent only in contralateral and not in ipsilateral overflow. As ipsilateral overflow was relatively rare for these tasks, this may simply be a floor effect. Several interactions involving hand order were revealed. A three-way interaction of Sex x Hand Order x Side, F(6,159) = 2 . 5 9 , ~ < .05, was significant only for SPl on univariate analysis. Boys who were tested first with the left hand were assigned higher contralateral overflow scores during this movement ( M = .56) than were boys who had been tested first with the right hand ( M = .26). Hand order did not play a role in ipsilateral overflow for boys (Lk= .34, RL = .35) and did not affect the performance of girls in either case. A two-way interaction of Sex x Hand Order was significant on univariate analysis only for SP2, F(6,159) = 2 . 3 9 , ~< .05. Again, boys tested first with the left hand received higher scores ( M = .63) than did boys tested first with the right hand ( M E .42). Girls did not show this order effect (Ms = .43 and .44, respectively). Finally, the interaction of Hand x Hand Order, 46,159) = 2 . 9 0 , ~< .05, was significant for task GM, with higher scores associated with the first hand tested, whether left (left hand = .21, right hand = .OS) or right (right hand = .17, left hand = .07). The two factors of primary interest were hand, F(6, 159)= 5.14, and the Hand x Side interaction, 4 6 , 159) = 5.74. Simple effects revealed hand differences in contralateral, F(6, 159) = 6.23, but not in ipsilateral ( F < 1) scores. Both main effect and interaction were significant at the univariate level for tasks P and SP2. For P, the average left-hand contralateral score ( M = .3 1) exceeded that of the right ( M = .lo), (simple effect: F(1,164) = 19.11). In contrast, the average righthand contralateral scorc for SP2 ( M = .44) exceeded that of the left ( M = .21), (simple effect: F(1, 164) = 13.08). Because of the importance of these findings, the scores were also subjected to the Wilcoxin Matched-Pairs Ranked-Signs (non-parametric) Test. Right-left difference scores wcrc ranked, ignoring signs.
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Then the sums of the ranks for positive and negative differences were calculated, and a test statistic 2 computed. This test confirmed that the hand differences obtained for P and SP2 were significant.
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Mirror vs. non-mirror contralateral overflow A breakdown of the frequency data for contralateral overflow into mirror (score = 2) and nonmirror (score = I) movements was then undertaken. The patterns of hand differences for P and SP2 were similar in both categories, and changed little across the three grades (see Table 1). The ratio of left to right hand overflow was 2.8: 1 for P, and 1:2.1 for SP2. Table 1. Frequency of mirror (score=2) and nonmirror (score=l) contralateral overflow in 176 right-handers. Grade 2 (n=63) Task: overflow type
Left
Grade 4
Grade 6 (n=58)
(n=55)
Right
Left
Right
Left
Right
Mirror(2) Nonmirror (1)
7 12
2 6
8 7
1 2
1 4
0 3
FTA: Mirror (2) Nonmirror (1)
12 24
15 29
11 15
7 19
1 5
3
L M : Mirror (2) Nonmirror (1)
5 0
4
2 0
1
3
0
0 0
1 0
LR: Mirror (2) Nonmirror (1)
6 4
I 2
2 0
2 1
0 0
1 2
SPl: Mirror (2) Nonmirror (1)
15 7
18 6
9 1
8 3
3 1
3 1
SP2: Mirror (2) Nonmirror (1)
8 6
9 13
1
6
I1 7
1 4
10
P:
8
4
Repetitive vs. finger-displacement tasks. Contralateral overflow scores were summed across the rapid repetitive tasks (P and FTA) and across the finger-displacement tasks (LM, L R , SP1, SP2). The two totals were submitted to a second MANOVA. In this analysis, hand was the only repeated measure. Two significant effects were revealed, one for grade, F(4, 326) = 13.00, and one for hand, F(2, 163) = 5.59, P < .005. Older children received lower scores on both measures, and hand differences were again evident (for finger-displacements, F(1, 164) = 7.62; for rapid repetitive movements, f'( 1,164) = 3.28, P = .072). As one would expect from the preceding analysis, higher scores were associated with the right hand for the former (left = .76, right = 1.06), and with the left hand for the latter (left = 34, right = .70). Visual examination of group means for individual tasks (Fig. 1) suggests that the second dependent measure reasonably represented performance across the
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four finger-displacement tasks. However, FTA did not conform to the expected pattern for a rapid repetitive task. In grade 2 (left = .76, right = .94) and in grade 6 (left = .12, right = .24), but not in grade 4 (left = .81, right = .60),the pattern of performance on this task was more suggestive of a finger-displacement task.
Left-handers The 19 left-handers were matched for grade, gender, and hand order with 19 right-handers randomly selected from the appropriate pool. A third MANOVA was conducted on this sample to investigate the effect of handedness. Three other variables (grade, hand, and side) were included in the analysis. Each task was entered as a separate dependent measure. Main effects for grade, F(12, 54) = 6.67, and side, F(6, 27) = 30.28, and an interaction between the two, F( 12, 27) = 26.00, were observed, confirming the results of the earlier analysis. Again, contralateral scores declined with age but ipsilateral scores did not. Ofgreater interest was the interaction of Handedness x Hand,F(6,27) = 2.86, p < .05. This was observed in the absence of a main effect for handedness ( F = 1.01) or hand ( F < 1). Univariate analyses revealed higher scores for P and L M with movement of the nondominant hand in both groups. Right-handers produced more overflow with left-hand movement (P: left = 1.3, right = .05; L M: left = .21, right = .05), and left-handers produced more overflow with righthand movement (for P: left = .05, right = .13; L M : left = ,18, right = .21). Left t o right hand ratios for P and SP2 were determined for the left-handed children. For task P, the frequency of left to right hand contralateral overflow was 1: 1.5, and for SP2,2.3: 1 (based on 2 instances of contralateral overflow with left hand movement, and 3 with right hand movement for P, 7 and 3 for SP2). Thus, although the hand difference for SP2 was not statistically reliable in this small sample, the data suggest a reversal for this task relative to P, as was observed for the right-handed children.
DISCUSSION Prior attempts to explain greater left-hand movement overflow by association with structural or functional hemispheric differences have typically ignored the several reports of greater right-hand movement overflow in this literature. This may have led to the premature assumption by many investigators that some property of one hemisphere accounts solely for the phenomenon. In the present study, a significant multivariate hand difference was observed for contralateral but not for ipsilateral overflow during rapid repetitive (left hemisphere) movements and discrete finger placement (right hemisphere) movements. Individual ANOVAs revealed greater overflow from left to right hands during forearm pronation/supination (a rapid repetitive task) and the reverse during finger-displacement movements. The latter was most evident for SP2, a finger-
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spreading task. The hand differences were seen in both mirror and nonmirror associated movements and were robust across the age range 7 to 11 years and across gender. Hand differences in contralateral overflow were also observed in a small sample of left-handed children. These children demonstrated greater right-hand overflow for two tasks (P and L M ) and greater left-hand overflow for SP2, although the latter was not statistically reliable. These findings are consistent with the view that both cerebral hemispheres contribute in a complementary fashion to movement, and (in an as yet unknown way) to contralateral overflow as well. Greater contralateral overflow appears to be associated with movement of the hand ipsilateral to the specialized hemisphere (e.g., the right hemisphere for SP2 and the left hemisphere for P in right-handers). This theory accommodates Cernacek's (1974) observation of greater right hand overflow for isolated finger flexions, a task which is similar to SP2 in that it requires careful finger positioning in a single, slow movement. Cernacek's finding derived from a careful study of EMG activity in adults, and cannot be accounted for by alternative theories currently available. Some problems for this interpretation should be considered. In the present study, reliable hand differences were not observed for all movements. Given the known sensitivity of measures of overflow to age and task effects (cf. Wolff et al., 1983),this may reflect a less than optimal sample size and/or the need for a more sensitive measure of overflow. It is more troublesome that a second rapid repetitive task -- finger-thumb alternation -- did not show the pattern of hand differences (even in lesser form) seen in the pronation/supination task. In fact, the pattern of scores for this task was more similar to SP2 than to P. Spatial elements in the task may have proven more problematic for the children than the requirements of rapid execution. Many of the children found it difficult to bring the fingers to one point in space and/or to determine the finger to move next. Given the close proximity and visual similarity of the fingers, these elements may require the use of internally generated relational cues (provided by the right hemisphere). Alternatively, some other movement classification scheme (based, for example, on proximal versus distal musculature, or perhaps on task familiarity or automaticity) may better predict the direction of greater overflow. For example, the direction of overflow for the finger alternation task might reverse with extended practice, as the sequence becomes automatized (and thereby increasing the relative contribution of the left hemisphere, cf. Goldberg & Costa, 1981). If the distinction is proximal/ distal, this manipulation should not affect the outcome. Inclusion of additional rapid repetitive tasks in future studies is needed. When the left-handed children were compared with a matched sample of right-handers, both samples showed greater contralateral overflow with movement of the nondominant hand for P and for L M , a finger-displacement task. Scores for L = M were strongly affected in the larger sample by hand order, such that greater overflow was typically associated with the first hand used,
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whether left o r right (see also Todor & Lazarus, 1985). This suggests that the hand difference for L-M was spurious, likely reflecting the fact that hand order was unevenly distributed in the small sample (due to random assignment). The absence of a hand difference for SP2 in the left-handed sample may be more important. Although a frequency comparison of left- and right-hand scores suggested that the direction of greater overflow reversed for tasks P and SP2 in left-handers as in right-handers, the hand difference for SP2 was not statistically reliable in the small sample. If one chooses to disregard the latter, the data support the preferential hand usage theory (i.e., that more overflow is associated with movement of the nondominant hand in this population). It is therefore possible that the pattern of overflow differs for left- and right-handers. Sample size will be a critical factor in any future investigation of this issue. The mechanism of asymmetrical overflow is still unclear. Why should there be greater contralateral overflow with movement of the hand ipsilateral to a specialized hemisphere? Perhaps the nonspecialized hemisphere (left or right) recruits or coactivates (Parlow & Kinsbourne, 1989)the specialized hemisphere, which results in a lowering of thresholds for movement of the other hand. The fact that cortical regions controlling hand movement in motor cortex are not directly (i.e. callosally) interconnected (Jenny, 1979), explains why heterologous and homologous muscles (cf. nonmirror and mirror movements) show the same directional effects. If use of the hand contralateral to thc specialized hemisphere does not lead to recruitment of the nonspecialized hemisphere, thresholds for movement of the other hand would be unaffected, and overflow between the hands less likely. This theory has some similarities to the second theory discussed in the introduction (cf. Kimura & Archibald, 1974), in that use of one hand may activate two hemispheres rather than one. However, it extends the principle to accommodate the specialized natures of both hemispheres. The co-activation theory can also account for the absence of observed hand differences in ipsilateral overflow. This finding presents a problem for theories that depend on the assumption that a more diffuse (or less refined) neural representation potentiates motor irradiation. To accommodate it, these theories must allow that distal (contralateral) brain regions are affected differentially and proximal (ipsilateral) brain regions are not (Liederman & Foley, 1987). The coactivation theory handles the problem easily, however, as recruitment (or not) of one hemisphere by the other should have little effect on cortical control of the active hand. The present findings suggest that hand differences in overflow vary with the nature of the intended movement and with handedness. A better understanding of these factors may well clarify a complex and difficult literature, and may also be expected to increase reliability for judgements of other neurological “soft signs” in the future (an issue not addressed here).
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REFERENCES Cernacek, J. (1961). Contralateral motor irradiation - cerebral dominance. Archives of Neurology, 4, 61-68. Connolly, K., & Stratton, P. (1968). Developmental changes in associated movements. Developmental Medicine and Child Neurology, 10, 49-56. Glass, G.V., Peckham, P.D., & Sanders, J.R. (1972). Consequences of failure to meet assumptions underlying the fixed effects analysis of variance and covariance. Review of Educational Research, 42, 237-288. Goldberg, E., &Costa, L.D. (1981) Hemispheric differences in the acquisition and use of descriptive systems. Brain andlanguage, 14, 144-173. Hiscock, M., Decter, M.H., Kinsbourne, M., & Mackay, M. (1987, February). Development of selective finger-movement control in normal children. Presented to the meeting of the International Neuropsychological Society, Washington, D.C. Jenny, A.B. (1979). Commissural projections of the cortical hand area in monkeys. Journal of Comparative Neurology, 188, 137-146. Kimura, D., & Archibald, Y. (1974). Motor functions of the left hemisphere. Brain, 92, 337-350. Kimura, D., & Vanderwolf, C.H. (1970). The relation between hand preference and the performance of individual finger movements by left and right hands. Brain, 93, 769774. Liederman, J., & Foley, L.M. (1987). A modified finger lift test reveals an asymmetry of motor overflow in adults. Journal of Clinical and Experimental Neuropsychology, 9, 498-510. Parlow, S.E., & Kinsbourne, M. (1989). Asymmetrical transfer of training between hands: Implications for inter-hemispheric communication in normal brain. Brain and Cognition 11, 98-1 13. Segalowitz, S.J., & Bryden, M.P. (1983) Individual differences in hemispheric representation of language. In S.J. Segalowitz (Ed.), Language functions and brain organization. (pp. 341-371). New York: Academic Press. Semmes, J. (1968). Hemispheric specialization: A possible clue to mechanism. Neuropsychologia, 6, 11-26. Todor, J., & Lazarus, J. (1985). Inhibitory influences on the emergence of motor competence in childhood. In L.D. Zaichowsky & C.Z. Fuchs (Eds.), Psychology of motor behavior: Development, control, learning andperformance (pp. 239-255). Ithaca, NY: Movement. Todor, J.I., & Lazarus, J.C. (1986). Exertion level and the intensity of associated movements. DevelopmentalMedicine and Child Neurology, 28, 205-212. Wolff, H., Gunnoe, E., & Cohen, C. (1983). Associated movements as a measure of developmental age. Developmental Medicine and Child Neurology, 25,417-429. Woods, B.T., & Teuber, H.L. (1978). Mirror movements after childhood hemiparesis. Neurology, 28, 1152-1158.
